Wireless networks are used to transmit data to wireless devices. Wireless networks often transmit data from a base station to a mobile device. To increase the amount of data a base station can transmit, the base station may transmit multiplexed data. The data may be multiplexed according to a standard such as the Worldwide Inter-operability for Microwave Access (WiMAX) standard that is based on the International Electrical and Electronic Engineers (IEEE) 802.16.x standard.
A base station operating according to the IEEE 802.16 standard uses orthogonal frequency division multiple access (OFDMA) signals to communicate data to mobile devices associated with the base station. Using OFDMA signals improves the rate of transmission between the base station and the mobile devices associated with the base station. An OFDMA signal is comprised of multiple subcarriers that are modulated at a symbol rate equal to the reciprocal of the frequency separation between two adjacent subcarriers. To reduce interference between multiple base stations transmitting OFDMA signals, neighboring base stations within a network are assigned different predetermined/predefined preamble sequences that have different data patterns. In one example, a base station operating in a 512-fast Fourier transform (FFT) mode may be assigned to one of 114 different preamble sequences. Adjacent base stations are assigned different preamble sequences to reduce interference between them.
The preamble sequence is sent from the base station as the first signal that the mobile device will receive upon the mobile device entering the broadcast range of the base station. The preamble sequence enables the mobile device to synchronize with the base station in order to transmit and receive frames of data with the base station. When the mobile device is within the range of more than one base station, the preamble sequences from different base stations may use the same frequencies and interfere with one another to cause a low signal to noise ratio (SNR). During signal acquisition with a particular base station, an OFDMA receiver in a wireless device does not know what preamble, frequency offsets, and/or timing offsets are associated with the signals transmitted by the base station. Therefore, the mobile device will need to account for uncertainties of preamble, frequency, and time at an SNR below zero. Searching for these unknowns at the same time may result in a high error rate and the correct preamble sequence may be difficult to determine. Therefore, a better way of identifying a preamble sequence would be desirable.